Fluidized bed powder coating process utilizing tribostatic charging

Abstract

A process for forming a coating on a conductive substrate, which comprises establishing a fluidized bed of a powder coating composition, in which the mechanism for particle charging is tribostatic charging immersing the substrate wholly or partly within the said fluidized bed, applying a voltage to the substrate for at least part of the period of immersion, whereby particles of the powder coating composition adhere to the substrate, withdrawing the substrate from the fluidized bed and forming the adherent particles into a continuous coating over at least part of the substrate.The process enables the coating of substrate areas which, because of the Faraday cage effect, are inaccessible in conventional electrostatic coating processes, and also enables the formation of thinner coatings than are obtainable by conventional fluidized-bed processes.

What is claimed is:  
     
       1. A process for forming a coating on a conductive substrate, which comprises establishing a fluidised bed of a powder coating composition, thereby effecting particle-charging of the powder coating composition by a mechanism consisting essentially of tribostatic charging of the powder coating composition in the fluidised bed, immersing the substrate wholly or partly within the said fluidised bed, applying a voltage to the substrate for at least part of the period of immersion, whereby charged particles of the powder coating composition adhere to the substrate, withdrawing the substrate from the fluidised bed and forming the adherent particles into a continuous coating over at least part of the substrate. 
     
     
       2. A process as claimed in claim  1 , wherein the substrate comprises metal. 
     
     
       3. A process as claimed in claim  1 , wherein the applied voltage is a direct-current voltage. 
     
     
       4. A process as claimed in claim  1 , for coating successive substrates in sequence, in which direct-current voltage is used and the polarity of the voltage applied to successive substrates is reversed from each substrate to the next so as to produce an alternating sequence. 
     
     
       5. A process as claimed in claim  4 , which is a continuous process in which a series of substrates of alternate polarities is transported through a fluidised bed established within a fluidising chamber having walls composed alternately, in the direction of travel of the substrates, of insulating sections and conducting sections. 
     
     
       6. A process as claimed in claim  1 , which comprises the simultaneous batchwise coating of one or more pairs of substrates disposed within a common fluidised bed, the substrates of each pair being charged by direct-current voltages to respectively opposite polarities. 
     
     
       7. A process as claimed in claim  1 , wherein the fluidised bed is established within an earthed vessel. 
     
     
       8. A process as claimed in claim  1 , in which one or more counter-electrodes are disposed within the bulk of the powder coating composition. 
     
     
       9. A process as claimed in claim  1 , wherein there is no earth connection to the substrate. 
     
     
       10. A process as claimed in claim  1 , wherein the substrate is wholly immersed within the fluidised bed. 
     
     
       11. A process as claimed in claim  1 , wherein there is no preheating of the substrate prior to immersion in the fluidised bed. 
     
     
       12. A process as claimed in claim  1 , wherein the powder coating composition is a thermosetting system. 
     
     
       13. A process as claimed in claim  1 , wherein the powder coating composition incorporates, by dry-blending, one or more fluidity-assisting additives. 
     
     
       14. A process as claimed in claim  13 , wherein the powder coating composition incorporates a combination of alumina and aluminium hydroxide as a fluidity-assisting additive. 
     
     
       15. A process for coating a conductive substrate which comprises an automotive or aerospace component, in which a first coating derived from a powder coating composition is applied by a process according to claim  1 , and thereafter a topcoat is applied over the powder coating. 
     
     
       16. Apparatus for use in a process as claimed in claim  1  for forming a coating on a conductive substrate, which comprises: 
       (a) a fluidising chamber;  
       (b) means for effecting fluidisation of a bulk powder coating composition within the fluidising chamber so as to establish a fluidised bed of the composition therein, thereby effecting particle-charging of the powder coating composition by a mechanism consisting essentially of tribostatic charging of the powder coating composition in the fluidised bed;  
       (c) means for immersing a substrate wholly or partly within the fluidised bed;  
       (d) means for applying a voltage to the substrates for at least part of the period of immersion, whereby the substrate becomes electrically charged so that charged particles of the powder coating composition adhere thereto;  
       (e) means for withdrawing the substrate bearing adherent particles from the fluidised bed; and  
       (f) means for converting the adherent particles into a continuous coating.  
     
     
       17. A substrate coated by a process as claimed in claim  1 . 
     
     
       18. A process as claimed in claim  1 , wherein the voltage applied to the substrate is such that the maximum potential gradient existing in the fluidised bed lies substantially below the ionisation potential gradient for gas in the fluidised bed. 
     
     
       19. A process as claimed in claim  1 , wherein a maximum potential gradient existing in the fluidised bed lies between 0.05 kV/cm and 10 kV/cm, both limits included. 
     
     
       20. A process as claimed in claim  19 , wherein a maximum potential gradient existing in the fluidised bed lies between 0.05 kV/cm and 5 kV/cm, both limits included. 
     
     
       21. A process as claimed in claim  20 , wherein a maximum potential gradient existing in the fluidised bed lies between 0.05 kV/cm and 1 kV/cm, both limits included. 
     
     
       22. A process as claimed in claim  1 , wherein the voltage applied to the substrate ( 6 ) lies between 5 kV and 60 kV, both limits included. 
     
     
       23. A process as claimed in claim  22 , wherein the voltage applied to the substrate ( 6 ) lies between 15 kV and 35 kV, both limits included. 
     
     
       24. A process as claimed in claim  22 , wherein the voltage applied to the substrate ( 6 ) lies between 5 kV and 30 kV, both limits included. 
     
     
       25. A process as claimed in claim  22 , wherein the voltage applied to the substrate ( 6 ) lies between 30 kV and 60 kV, both limits included. 
     
     
       26. A process as claimed in claim  1 , wherein the particles of the powder coating composition vary in size between 1 and 120 microns, both limits included. 
     
     
       27. A process as claimed in claim  26 , wherein the particles vary in size between 15 and 75 microns, both limits included. 
     
     
       28. A process as claimed in claim  27 , wherein the particles vary in size between 25 and 50 microns, both limits included. 
     
     
       29. A process as claimed in claim  27 , wherein the particles vary in size between 20 and 45 microns, both limits included. 
     
     
       30. A process as claimed in claim  1 , wherein the substrate receives a continuous coating of thickness between 5 and 200 microns, both limits included. 
     
     
       31. A process as claimed in claim  30 , wherein the substrate receives a continuous coating of thickness between 5 and 100 microns, both limits included. 
     
     
       32. A process as claimed in claim  30 , wherein the substrate receives a continuous coating of thickness between 10 and 150 microns, both limits included. 
     
     
       33. A process as claimed in claim  32 , wherein the substrate receives a continuous coating of thickness between 20 and 100 microns, both limits included. 
     
     
       34. A process as claimed in claim  33 , wherein the substrate receives a continuous coating of thickness between 60 and 80 microns, both limits included. 
     
     
       35. A process as claimed in claim  33 , wherein the substrate receives a continuous coating of thickness between 80 and 100 microns, both limits included. 
     
     
       36. A process as claimed in claim  31 , wherein the substrate receives a continuous coating of thickness between 50 and 150 microns, both limits included. 
     
     
       37. A process as claimed in claim  32 , wherein the substrate receives a continuous coating of thickness between 15 and 40 microns, both limits included. 
     
     
       38. A process as claimed in claim  1 , wherein less than 10 mA flows in the substrate. 
     
     
       39. A process as claimed in claim  38 , wherein less than 5 mA flows in the substrate. 
     
     
       40. A process as claimed in claim  39 , wherein less than 1 mA flows in the substrate. 
     
     
       41. A substrate coated by a process as claimed in claim  18 . 
     
     
       42. The process of claim  8 , wherein the counter-electrodes are earthed.